how can I open an hdr in blender
Blender is not an image viewer application, you can't just use Blender to directly open image files from File > Open

You also can't just "view" images directly nor can you expect it to just open a spherical image as display it as a panorama viewer.

You can however use the builtin UV Image Editor to view regular images and use a workaround to view spherical 360 panoramas.

Starting a few versions ago Blender added support for world preview as viewport background. As a workaround you can use that as sort of a spherical image viewer. It will be obscured by the 3D View interface (grids, cursor and objects), but serves as a basic viewer.

With Cycles as your current render engine create a new World datablock from the Properties Window. Add a new Environment texture to its color input.

Pick the Open image button and browse your file system to the desired image file (JPG, PNG, or HDR formats).

Pick the image then in the Properties Shelf over the 3D View (N key) under View tab activate the World Background option if disabled.

You should now be able to use the 3D Viewport as a basic spherical 360 panorama viewer.